1. Field of the Invention
The present invention relates to miniature radio apparatuses, which are capable of being mounted on the human body or carried by hand and which transmit or receive electric waves.
2. Description of Related Art
Recently, with the increasing trend of intensification of information, the spread of miniature radio apparatuses which allow the user thereof to communicate with anybody at anytime and in any place has been accelerated in every country. Particularly, the pager has made a rapid growth on a global level and there are various kinds of pager configurations including box, pencil and watch types.
In these radio apparatuses, the built-in antenna is an indispensable element and the loop antenna as representing it is chiefly used. Of these apparatuses, the watch type radio apparatus cannot have a sufficient antenna length so that it uses a ferrite antenna as in the case of an AM radio or a conductor to be inserted into a watch band so as to be put to practical use as an antenna.
In FIG. 1, there is shown the principle of a loop antenna.
The tuning to the reception frequency is made through the inductor and the capacitor interposed between conductors a and e shown in FIG. 1. The wave is then supplied from taps a b to a reception circuit. As is well known, the reception sensitivity is determined by the material of the conductor, the quality of the condenser C, and the aperture area of the antenna. The larger the aperture area the higher the antenna gain.
Similarly, an equivalent circuit at the time when a watch band is used as an antenna is shown in FIG. 2. Reference numeral 20 designates the body of a miniature radio apparatus, reference numeral 21 designates a clock circuit and a control section, reference numeral 22 designates a reception circuit and reference numerals 23 and 24 designate band halves each in the form of a belt made of a conductive material such as copper and partly insulated by an insulating material such as a plastic to prevent it from contacting the human body.
Reference letters e and g designate connections of the band halves forming a band and the body 20 of the miniature radio apparatus, at which connections, the band halves are mechanically connected with the miniature radio apparatus, and an electric wave is introduced electrically to the reception circuit 22 through a matching circuit.
It should be noted that in FIG. 2, the matching circuit determines the resonant frequency through the capacitor C, and the frequency is drawn from taps of the terminals a and b.
Reference letter f is a clamp for electrically and mechanically connecting the band halves 23 and 24 together. This clamp is made of a material having a low level of high frequency resistance and a high degree of durability.
Thus, the loop antenna has conventionally been formed in the above manner. However, the antennas using this band have had the following various problems.
Thus, in the case of miniature radio apparatuses, particularly the wrist watch type radio apparatus, if the wrist band is used as an antenna, the following problems have arisen. That is:
Since the open area is small, sufficient antenna gain cannot be obtained. Especially when a FM multiplex apparatus having a low level of frequency is used, the antenna gain lowers. PA1 The open space surrounded by the band changes depending on the physical size of the user which results in changing the antenna gain. Particularly, when the user is a female or a child, it remarkably deteriorates. PA1 The apparatus and the band have to be electrically coupled, so that specially-designed band is required. Therefore, the user can not use a band to his liking. PA1 If contact points of the band deteriorate or have contact resistance, the antenna gain deteriorates. PA1 Since there are contact parts between the apparatus and the band, it is difficult to, provide a sufficiently waterproof device. PA1 Since the antenna material is put in the band, the band looses its flexibility and becomes uncomfortable. PA1 Since the band has no flexibility, the durability of the band is poor. PA1 Since the antenna is of the magnetic field type, the resonance quality becomes high so that it becomes necessary to make an automatic frequency adjustment depending on the size of the user's wrist and the circuit and the system becomes complicated. PA1 Since the band has electrically connected parts, a particular,connecting metal becomes necessary and the user feels it, uncomfortable and difficult to wear.
In the above-described miniature radio apparatus, it has been thought that it is not possible to miniaturize the apparatus while keeping a high gain. Such tendency has been particularly remarkable in the case of the wrist watch for use with a FM frequency band having a large wavelength. To take one example, in the conventional FM multiplex pager, antenna gain thereof was, as compared with the 1/2.gamma. dipole, about -30 dB, namely, the relative gain was about -30 dBd, and the performance of the apparatus was not good for its outfit. For this reason, it has been usual that the reception sensitivity of the whole apparatus became worse resulting in the deterioration of the communication quality thereof.
Further, although an electric field type antenna having one of its pair of terminals connected to the human body when in use, when it is used with a miniature radio apparatus for receiving a long wavelength FM band, it has been usual that since the area of the circuit is small, the false ground for constructing the electric field type antenna is insufficient and it is not possible to obtain a favorable gain.
Although an example of paging for receiving a radio wave has been described above, the problem involved therein is the same as in the case of a miniature security radio transmitter/receiver which is capable of transmission by pressing an emergency button.